High graphite N content in nitrogen-doped graphene as an efficient metal-free catalyst for reduction of nitroarenes in water

Article abstract of DOI:10.1039/c6gc00222f

Four kinds of nitrogen-doped graphene (NG) as metal-free catalysts are synthesized by a one-step hydrothermal reaction and thermal treatment using graphene oxide and urea as precursors. It is found that the reduction of nitroarenes can be catalyzed by using a low NG loading and a small amount of NaBH₄ in water with high yield. The type of nitrogen species in NG has an important effect on the reduction reaction. The NG catalyst containing the most graphite N shows the highest catalytic activity during reduction of nitroarenes, which demonstrates that the graphite N of NG plays a key role in impelling this reaction. The reaction mechanism is proven by GC-MS experiments, and DFT calculations reveal the reasons for the graphite N showing better catalytic activity. It is worth noting that no dehalogenation phenomenon occurs during the reduction process for halogen-substituted nitroarenes in contrast to conventional metal catalysts. In addition, the NG catalyst can be simply recycled and efficiently used for eight consecutive runs with no significant decrease in activity.

Full text of DOI:10.1039/c6gc00222f

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Journal Name
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ARTICLE
High Graphite N Content in Nitrogen-Doped
Graphene as an Efficient Metal-free Catalyst for
Reduction of Nitroarenes in Water
Cite this: DOI: 10.1039/x0xx00000x
a†
a†
b
c
a
Fan Yang, Cheng Chi, Chunxia Wang, Ying Wang * and Yongfeng Li *
Received 00th January 2012,
Accepted 00th January 2012
Four kinds of nitrogen-doped graphene (NG) as metal-free catalysts are synthesized by a one-
step hydrothermal reaction and thermal treatment using graphene oxide and urea as precursors.
It is found that the reduction of nitroarenes can be catalyzed by using low NG loading and small
DOI: 10.1039/x0xx00000x
www.rsc.org/
4
amount of NaBH in water with high yield. The type of nitrogen species in NG shows an
important effect on the reduction reaction. The NG catalyst containing the most graphite N shows
the highest catalytic activity during reduction of nitroarenes, which demonstrates that the
graphite N of NG displays a key role in impelling this reaction. The reaction mechanism is proved
by GC-MS experiment, and DFT calculation reveal the reason for the graphite N shows better
catalytic activity. It is worth to note that no dehalogenation phenomenon occurs during reduction
process for halogen substituted nitroarenes in contrast to conventional metal catalysts. In
addition, the NG catalyst can be simply recycled and efficiently used for eight consecutive runs
with no significant decrease in activity.
1
0ꢀ12
Introduction
readily available properties.
As one of the most remarkable carbon materials, graphene,
consisting of a single or few layers of graphite, acts as a
scaffold which attracts encouraging attention due to its
The catalytic reduction of nitroarenes to the corresponding
amine is considerable in organic synthesis because it is one of
the atomic efficiency methods to produce intermediates or the
key precursors of pharmaceuticals, polymers, pesticides,
13ꢀ15
excellent physical, chemical and mechanical properties.
Moreover, it is well known that doping nitrogen heteroatoms is
an efficient way to tailor its electric properties and expand its
applications, such as biosensors, transparent counter electrodes,
1
explosives, fibers, dyes and cosmetics. For their importance, a
plentiful of metal catalysts, including Pd, Au, Cu, Fe, Ni, Sm,
Ag, and Zn to catalyze reduction of nitroarenes have been
electrochemical sensors, supercapacitor electrodes and
2
ꢀ7
developed. Unfortunately, some metalꢀbased catalysts suffer
from low tolerant toward substituted nitroarenes, leading to
undesired byproducts and difficult purification. Therefore,
bimetallic nanoparticle catalysts are developed for reduction of
nitroarenes to enhance the reaction activity and selectivity due
16ꢀ20
catalysts.
Therefore, numerous methods have been explored
to synthesize nitrogenꢀdoped graphene, including direct
synthesis: CVD, solvothermal, arcꢀdischarge approach in the
21ꢀ24
presence of C, Nꢀcontaining precursors;
and postsynthesis
treatment: thermal and plasma treatment of synthesized
8,9
to its synergistic effects. Recently, metalꢀfree catalysts, such
as carbon materials, have attracted more and more attention for 27
25ꢀ
graphene or graphene oxide in an Nꢀcontaining environment.
catalysis of organic molecule transformation reactions, due to
its cheap, stable, biocompatible, environmentꢀfriendly and
In the field of catalysis, the grapheneꢀbased materials are
found to catalyze various types of organic molecule
transformation, such as oxidation reaction, reduction reaction,
a
State Key Laboratory of Heavy Oil Processing, China University of
2
8,29
Petroleum, Beijing, Changping 102249, China. Phone/Fax: +86-010-
ring opening reaction and Michael addition.
The reduction
8
9739028, yfli@cup.edu.cn.
b
of nitroarenes to the corresponding amine is considerable in
organic synthesis because the amine is the key intermediates of
pharmaceuticals, polymers, pesticides, explosives, fibers, dyes
Beijing National Laboratory for Molecular Sciences, Key Laboratory of
Analytical Chemistry for Living Biosystems, Institute of Chemistry, the Chinese
Academy of Sciences, Beijing 100190, China.
c
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute
3
0
and cosmetics. Although, the carbon materials, such as Nꢀ
doped carbons, reduced graphene oxide and NG have been used
as catalysts for reduction of 4ꢀnitrophenol, the high loading of
of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
China. ywang_2012@ciac.ac.cn
†
These authors contributed equally.
‡
Electronic supplementary information (ESI) available: DFT calculation detail,
1
31ꢀ34
spectroscopic data ( H NMR) of products.
catalyst and large amount of reductant are needed.
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Therefore, the development of highly effective and handleꢀ separate solid from liquid. The yellow solid was washed with
convenient catalysts, less toxic solvent, low temperature and HCl (1 mol/L) one time and deionized water four times to
high selective reaction system for reduction nitroarenes reaction remove oxidant agents. Then the remaining solution was
is still highly desirable.
dialyzed for one week to eliminate excess ions. The obtained
In this work, we prepared high graphite N content of graphite oxide was dispersed in water and subsequently
nitrogenꢀdoped fewꢀlayered graphene materials by a oneꢀpot ultrasonicated (120 W, 40 kHz) for 2 h. The final product was
hydrothermal method. It is found that the reduction of aromatic obtained by freeze dried.
nitro compounds could be triggered by a small amount of
NaBH4 and low loading of high graphite N content NG in
Synthesis of NG sheets and reduction of GO
water at room temperature. The catalyst show good tolerance to Four kinds of NG catalysts were prepared by a oneꢀstep
variety of different functional groups, and various kinds of hydrothermal reaction of graphene oxide with urea and thermal
3
6
substituted nitroarenes could be reduced with excellent yields treatment. Typically, 50 mg GO together with 1.5 g urea
and high selectivity. More importantly, the proportion of doped (1:300), was diluted in 35 ml deionized water under sonication
nitrogen species could significantly affect the NG catalytic for 1 h. The solution was sealed in 50 ml Teflonꢀlined autoclave
o
behavior, which are demonstrated by the DFT calculation. The and maintained at 180 C for 12 h. The black solids (Nꢀdoped
reaction intermediates are studied by GCꢀMS experiment, and graphene sheets) were filtered then separated into four sections:
the reaction mechanism is proposed according to the GSꢀMS the first section was washed with hydrochloric acid (1 mol/L)
results. Moreover, the catalyst is very stable and easily one time and distilled water four times to remove the excess
recycled, which could be recycled for eight times without urea. The collected sample was freeze dried overnight for
dramatic decline in catalytic activity.
further use, denoted as NGꢀ1. The other three NG catalysts are
o
o
obtained by thermal treatment of the samples at 700 C, 800 C
o
Experimental
and 900 C for 4 hours with Ar continuously flowing, denoted
as NGꢀ700, NGꢀ800 and NGꢀ900, respectively. The thermal
treated samples are prepared by directly filtrated without being
General information
All the chemicals are used as received without further washed after hydrothermal treatment of the GO and urea.
purification: aromatic nitro compounds (Aladdin industrial Afterwards they were washed with deionized water several
corporation); urea (Sinopharm Chemical Reagent Beijing Co., times and freeze dried overnight. For comparison, reduced
Ltd); graphite powder (200 mesh), silica gel (Alfa Aesar, graphene oxide (rGO) was synthesized similarly by
o
Johnson Matthey Company); KMnO , H PO , H SO , H O , hydrothermal method in 180 C for 12 h, following by washing
4
3
4
2
4
2
2
NaOH, ethanol, acetic ether, hydrochloric acid (Beijing with deionized water and freezeꢀdrying overnight.
Chemical Works). The morphology and the size of the NG
Catalytic reduction of nitroarenes
samples were characterized by transmission electron
microscopy (TEM, FEI, F20) combined with an energy In a typical catalysis reaction, 2 mg NGꢀ1 was mixed with 0.5
dispersive Xꢀray spectroscopy (EDS) at an acceleration voltage mmol pꢀnitrchlorobenzene (40 mg) and 1 ml water, then stirred
of 200 kV. Xꢀray diffraction (XRD, Bruker D8 Advance for 1ꢀ2 min for thoroughly mixing. 10 equiv. of NaBH (2.5
4
Germany) was applied to characterize the crystal structure of mmol/mL) 2mL was added dropwise into the above solution
the hybrid materials, and the data was collected on a Shimadzu under magnetic stirring at room temperature. The reaction was
XDꢀ3A diffractometer using Cu Kα radiation. The Xꢀray monitored by thin layer chromatography (TLC). After the
photoelectron spectroscopy (XPS, Thermo Fisher KꢀAlpha reaction, the crude product was isolated by extracting four
American with an Al Ka Xꢀray source) was used to measure the times by ethyl acetate. The aqueous phase with NG catalyst was
elemental composition of samples. The Hꢀnuclear magnetic washed with water and alcohol twice, respectively. It was
1
resonance ( HꢀNMR) was recorded on the JNMꢀLA300FTꢀ centrifuged (3100 rpm, 3 min) for separation each time, and
o
NMR (300 MHz, 400 MHz) for checking the final product. The dried at 60 C for the next cycles. The organic phase was
reaction intermediates were analyzed by using a BRUKER purified by two methods. Workup A, the saturated hydrochloric
SCION TQ GCꢀMS.
ethyl acetate solution was added to the organic phase, and a
white precipitate (the aminehydrochloride) was formed
immediately. The hydrochloride was removed by filtration, and
Synthesis of Graphene Oxide
Graphene oxide (GO) was synthesized from graphite powder washed with ether to give the yield. Workup B, the organic
3
5
according to a modified Hummer's method. Typically, phase was concentrated, and the residue was purified by a
graphite powder (0.5 g) and KMnO (3 g) were initially put into column chromatography (petroleum ether/ethyl acetate
4
a
roundꢀbottom flask. Then
a
mixture of concentrated mixture) to give the isolated yield. With the isolated product
1
H SO /H PO (60 mL:ꢁ6.6 mL) was added. The mixture was yield in hand, the structure of products was identified by H
2
4
3
4
stirred at 50 °C for 12 h. After cooling down to room NMR.
temperature, the mixture was transferred into an ice bath ( 40
mL). 30% H O ( 2.5 mL) was added drop wise with stirring Results and discussion
for 0.5 h. The mixture was centrifuged (4000 r/min, 5 min) to
∼
∼
2
2
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6GC
A
00
R
2
T
2
I
2
C
F
LE
Four kinds of NG catalysts have been prepared by a oneꢀstep
Further insights of the structural properties of NG are obtained
hydrothermal reaction of graphene oxide with urea and thermal from XRD characterization, as shown in Fig. 2a. The key
o
treatment. The morphologies of the resultant NG and GO are indicator of GO is centered at 10.6 , being indexed to (001)
characterized by SEM and TEM images, as shown in Fig. 1. plane, corresponding to
a
0.83 nm interlayer space,
The NG nanosheets showed a typical crumpled surface with demonstrating the insert of oxygenꢀcontaining functional group
o
random stacking, which is possibly attributed to the defective to form the graphene oxide. In the case of NG, the peak at 10.6
o
structure formed upon exfoliation and the presence of foreign disappears and a small peak at 26.5 index to (002) plane of
nitrogen atoms. The highꢀresolution TEM image further graphite emerged after hydrothermal and thermal process.
indicates that these nanosheets consist of 3ꢀ10 layers. The These results indicate that the wellꢀordered graphene with 0.34
interlayer distance is averagely
∼
0.37 nm, as shown in Figs. nm spacing has been recovered after reduction process. The
0.34 four types of NG catalysts show no significant difference,
1
b2ꢀe2, similar to the interlayer distance of graphite (
∼
1
7
nm). It can be seen from SEM image that compared with the which demonstrates that the different temperature has no
silk veilꢀlike morphology of GO, partially aggregated and significant influence on nitrogenꢀdoped graphene crystalline
crinkled structure seen in Figs. 1bꢀe was obtained at the end of form. However, with elevated temperature, the peak is shifted
doped graphene sheets of NGꢀ1, which is in accordance with to a slightly higher 2θ degree, implying a better elimination of
previous report that the GO layers would be refabricated when surface functional groups and that nitrogen atoms have been
1
1, 38
they were thermal treated and influenced by the heteroatom successfully doped into the graphitic system.
doping process.
3
7
Fig. 1 SEM images of GO (a), NG-1 (b), NG-700 (c), NG-800 (d), NG-900 (e); TEM images of GO (a1), NG-1 (b1), NG-700 (c1), NG-800 (d1), NG-900 (e1); HRTEM images
of GO (a2), NG-1 (b2), NG-700 (c2), NG-800 (d2), NG-900 (e2).
3
9
associated with disordered samples or graphene edges. It is
well known that the integrated intensity ratio of D band versus
G band ( D
and disorder level in the graphitic carbon layers. The ID
values of GO, NGꢀ1, NGꢀ700, NGꢀ800, NGꢀ900 are 0.90, 1.07,
.14, 1.08 and 1.05, respectively, indicating that more defective
I /IG) is a significant parameter of reflecting the defect
4
0
/
IG
1
sites have been created through the incorporation of nitrogen
atoms into the carbon network. Moreover, the I /I value of NG
D
G
shows decrease with the temperature increasing, which may be
due to the fact that NG is partial reduced by decrease of
nitrogen atom under high temperature.
The element analysis of the NG samples has been further
Fig. 2 (a) XRD patterns of GO and NG samples. (b) Raman spectra of GO and NG
samples.
4
1
Raman spectroscopy is an important tool for identifying the
structure of graphene to distinguish the interaction between characterized by XPS (Fig. 3a). There exist three peaks in the
molecules, the number of layer and doping status of graphite. XPS spectrum, corresponding to C1s, O1s and N1s at about
The Raman spectra of GO and NG are displayed in Fig. 2b. The 285.08, 399.08 and 532.10 eV, respectively. The content of
ꢀ
1
G band (~1599 cm ) is commonly observed for all graphitic oxygen in each sample is 10.17%, 7.16%, 6.33% and 4.23%,
structures due to the E2g vibrational mode present in the sp2
ꢀ
1
bonded graphitic carbon and the D band (~1344 cm ) is
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Table 1. The peak positions and relative atomic ratios of N species for NGꢀ1, NGꢀ700, NGꢀ800 and NGꢀ900 samples based on
a
XPS analysis
Pyridinic N
Pyrrolic N
Ratio
Graphitic N
Pyridinic Nꢀoxide
BE
eV)
Ratio
(%)
BE
BE
Ratio
(%)
BE
Ratio
(%)
Catalyst
N-1
Total N ratio (%)
(
(eV)
(%)
(eV)
(eV)
398.4
398.5
398.5
398.1
1.27
3.04
2.46
0.98
399.4
399.5
399.8
399.3
2.71
400.3
4.26
1.39
2.03
1.05
402.6
0.59
1.36
1.90
0.40
8.83
8.03
6.94
3.60
N-700
N-800
N-900
2.40
0.56
1.17
400.7
400.6
401.2
402.1
401.8
404.3
a
Hydrogen is not taken into account for the calculation.
Fig. 3 XPS survey of NG samples (a). N scans of NG-1 (b), NG-700 (c), NG-800 (d), NG-900 (e). Configuration of four types of N species (f). Nitrogen content of each
type of N species in different samples (g).
and the contents of N element in NGꢀ1, NGꢀ700, NGꢀ800, NGꢀ oxide bonded to two carbon atoms and one oxygen atom, which
4
2
9
00 are 8.83%, 8.03%, 6.94%, 3.60%, respectively, as seen in is also located at the edges of graphene plane (Fig. 3f).
Fig. 3a and Table 1. The high resolution N1s XPS spectrum of Moreover, the analysis of the N1s peak provides the relative
NG shown in Figs. 3bꢀe demonstrates that the binding energy atomic ratios of each type of N species are summarized in
around 398, 399, 401 and 402 eV is attributed to pyridinic N, Table 1 and Fig. 3g. For NGꢀ1, the main type nitrogen is
3
6
pyrrolic N, graphitic N and pyridinic Nꢀoxide, respectively.
graphitic N, which occupies 48% of total N atoms. As for the
Pyrrolic and pyridinic N are always located at the margin of annealing samples, the large amount of urea can react with NGꢀ
graphene, bonded with carbon atoms, forming fiveꢀmembered 1, which can be used to explain the decrease in graphitic N after
ring and six membered ring. Graphitic N substitutes carbon thermal treatment. As for why hydrothermal treatment of the
atoms inside the graphene sheet, and it can be found at either NGꢀ1 and urea can decrease the graphitic N content is unclear.
the edge or the center of graphene framework. Pyridinic Nꢀ It can be seen that, with elevated annealing temperature from
4
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o
o
7
00 C to 800 C, the contents of the pyridinic and the pyrrolic completely by using 2mg NGꢀ1 (entry 2). Among various kinds
N show a decrease, and the graphitic N shows an increase with of reductants, the weak reductive reductants, such as hydrazine
4
3
enhancing the temperature. It is probably due to its thermal hydrate, hydrogen, ethylene glycol, phenyldimethylsilane,
stability at high temperature. When annealing temperature exhibit lower reactivity compare to that of NaBH (entries 3ꢀ6).
4
o
o
increased from 800 C to 900 C, the graphitic N of NGꢀ900 are Although the amount of N atom in NGꢀ700 catalyst is similar to
decreased compared with that of NGꢀ800. However, the ratio of the case of NGꢀ1, the content of graphite N in NGꢀ700 is lower
the graphitic N in total N content does not decrease. This result than that in NGꢀ1, resulting in low yield of 4ꢀchloroaniline
may be due to the N species which have been directly removed (entry 7). It is interesting to note that the NGꢀ800 catalyst with
from the graphene sheet, leading to the decrease of the total N lower content of N atom shows better activity than NGꢀ700
3
7
content. The phenomenon for the first increase and then (entry 8), properly due to the higher graphite N content in NGꢀ
decrease of the graphitic N with increasing the temperature is 800 than the case of NGꢀ700. Among all the NG catalysts, the
4
4
also previously reported. It is worth to note that the graphitic NGꢀ900 catalyst shows a lower reactivity, however, the yield of
N content in NGꢀ1 is obviously larger than those in NGꢀ700, 4ꢀchloroaniline is not reduce dramatically compare to the case
NGꢀ800 and NGꢀ900, which indicates the hydrothermal using NGꢀ700 as catalyst (entry 9). These results indicate that
treatment can probably form dominant stable graphitic N.
graphite N in NG catalysts is critical for impelling the reaction
rather than the amount of N atom in NG catalysts. In order to
identify our prediction, the amounts of NGꢀ800 and NGꢀ900
catalysts are further increased to equal the Nꢀcontaining of NGꢀ
Table 2. Evaluation of reaction conditions for the reduction of
a
4
ꢀchloronitrobenzene
1
, producing 4ꢀchloroaniline in 97% and 95% yields for 6 h
(
entries 10 and 11). The difference of activity is due to the
different amount of graphite N in each catalyst sample.
Moreover, for the purpose of comparison, a series of control
experiments, such as without catalyst, using GO, rGO, C N
3 4
and isoquinoline as catalysts have been performed. The results
indicate that GO and rGO show a lower reactivity, producing 4ꢀ
b
Entry
Catalyst
reductant
Time
12
3
Yield
90
c
1
2
3
NGꢀ1
NaBH
4
4
NGꢀ1
NGꢀ1
NaBH
98
NH
2
NH
2
·H
2
O
4
73
no
chloroaniline in 37
demonstrating that O atom contributes little to the catalytic
reduction reaction. In the cases of using of C , isoquinoline
or without catalyst, the reaction does not proceed (entries 14ꢀ
6). On the other hand, the high catalytic activity of NG is not
% and 64% (entries 12 and 13),
4
5
NGꢀ1
NGꢀ1
H
2
24
24
reaction
no
reaction
OHCH
2
CH
2
OH
N
3 4
6
7
8
9
NGꢀ1
PhMe
2
SiH
24
12
12
12
6
36
53
70
42
97
95
37
64
1
NGꢀ700
NGꢀ800
NGꢀ900
NaBH
NaBH
NaBH
NaBH
NaBH
NaBH
NaBH
4
4
4
4
4
4
4
caused by the defect sites on the surface of NG due to the fact
that the catalyst NGꢀ700 with the largest defective sites (Fig. 2b)
is not the best catalyst. These results strongly demonstrate that
the NG catalyst is indispensable for hydrogenation of
nitroarenes.
Moreover, we have further extended the NGꢀ1 catalyst to
various nitroarenes to examine the generality of the reaction.
The scope and limitations of NGꢀ1 catalyzed reduction of
d
1
0
1
2
3
NGꢀ800
NGꢀ900
GO
e
1
1
1
6
24
24
rGO
no
1
4
no catalyst
NaBH
4
24
reaction
1
1
5
6
C
3
N
4
NaBH
4
4
24
24
trace
trace
nitroarenes into amines by NaBH
Table 3. For nitrobenzene, the aniline is formed in 85% yield
entry 1). It is noteworthy that chloro, bromo and iodoꢀ
substituted nitroarenes are reduced without undergoing any
dehalogenation, affording the corresponding products in high
yields (entries 2ꢀ4). The reactions of substrates bearing an
electronꢀdonating group at R produce the corresponding
products in excellent yields (entries 5ꢀ7). Substrate 4ꢀ
nitroanisole with a strong electroꢀdonating group afforded the
desired products in moderate yield by using 20 equiv. NaBH4
4
in water are summarized in
Isoquinoline
NaBH
(
a
Standard reactions conditions: 0.5 mmol 4ꢀ
chloronitrobenzene, 2 mg of catalyst, 3 mL H O, 5 mmol (10
equiv.) reductant under room temperature. Isolated Yield. 1
mg of catalyst was used. 2.5 mg NGꢀ800 was used. 4.9 mg
NGꢀ900 was used.
2
b
c
d
e
The NG catalyst is used to catalyze the hydrogenation of
nitroarenes. Reductants and catalysts were preliminary
investigated on the hydrogenation of 4ꢀchloronitrobenzene for
the formation of 4ꢀchloroaniline at room temperature in water,
and the results were summarized in Table 2. To our great
delight, when using 1 mg NGꢀ1 catalyst to catalyze the
hydrogenation 0.5 mmol of 4ꢀchloronitrobenzene with 5 mmol
NaBH in H O, 90% yield of 4ꢀchloroaniline is obtained in 12 h
(
entry 8). In addition, electroꢀwithdrawing groups, such as
carboxyl and cyano functionalities in nitroarenes remain intact
by using the NG catalyst, indicating a highly chemoselective
process (entries 9 and 10).
The recyclability of NGꢀ1 has been evaluated by using 4ꢀ
chloronitrobenzene as test substrate. After completion of the
reaction, the catalyst was filtered from the reaction and washed
4
2
(
entry 1). The hydrogenation of 4ꢀchloronitrobenzene proceeds
by H O for next run. This process is repeated for eight cycles,
2
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giving all cycles with excellent yields when prolonging the hydroxylamine yield the azoxy compound, which is reduced in
reaction time to 6 hours (Fig. 4). a series of consecutive steps to the azo, hydrazo and aniline
A probably reaction mechanism for NG catalyst with high compounds, as shown in the condensation route of Fig. S2.
catalytic activity during the reduction of 4ꢀchloronitrobenzene However, we cannot exclude the reaction proceeded in the
4
5
has been proposed in Fig. 5. NG plays the positive role to direct route of Fig. S2. To investigate the influence of the
improve the catalytic activity, which is ascribed to the carbon doped N atoms, the configurations of four kinds of nitrogenꢀ
atoms next to the doped N atoms on NG surface can be doped graphene together with the adsorbed nitrobenzene (as a
activated. Moreover, the absorption of reactants on the surface representative of nitroarene) were optimized by DFT. The most
2
0
ꢀ
of NG is a critical step for catalyzing the reactions. First BH4 stable four kinds of representative structures are shown in Fig.
ions can be adsorbed on the surface of NG catalyst in solution. 6. The computational details are described in supporting
Meanwhile, nitroarenes are also adsorbed onto the NG surface information. According to the Fig. 6, it is found that the parallel
and this process is reversible (adsorption accompanied by adsorption with an AB stacking structure is more stable than
desorption). Once both the substrates are chemisorbed onto the vertical adsorption due to the lower adsorption energy.
NG surface there is a hydrogen transfer to the nitroarenes. To Furthermore, compared to these four NG catalysts, the graphitic
obtain the reaction pathway involved in NGꢀcatalyzed NG exhibits the lowest adsorption energy and the longest NꢀO
reduction reactions, the intermediate state has been analyzed by bonds, indicating the highest catalytic activity, which is in good
the GCꢀMS, as shown in Fig. S1 in which the hydroxylamine, agreement with the experimental observations. This good
hydrazine and diazene oxide three intermediates have been performance is attributed to the weak conjugation and higher
detected. Therefore, the reaction mechanism may be concluded positive charge density on the neighbour carbon atoms,
that the aromatic nitro compound is reduced to the nitroso activated by the doped N atoms, being consistent with the other
2
0
compound and then to the corresponding hydroxylamine. Then, theoretical results.
one molecule of the nitroso compound and a molecule of the
a
Table 3. Catalytic reduction of nitroarenes into amines by NaBH4 catalyzed by NGꢀ1
b
Entry
Reactant
Product
T (h)
3
Yield (%)
1
2
3
4
5
6
7
85
98
90
82
95
93
93
80
85
63
3
5
15
8
6
2
c
8
9
c
9
6
c
1
0
15
a
Standard reactions Conditions: 0.5 mmol nitroarene, 2 mg of NGꢀ1, 3 mL H O, 5 mmol (10 equiv.) NaBH under room
2
4
b
c
temperature. Isolated yield. 10 mmol NaBH was used.
4
6
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Fig. 5 A possible reaction mechanism for the reduction of 4-chloronitrobenzene
catalyzed by the NG-1 catalyst and sodium borohydride.
Fig. 4 The catalyst reusability for the reduction of 4-chloronitrobenzene.
Fig. 6 The structures of nitroarenes adsorbed on NG: adsorbed at the (a) graphitic N, (b) pyridinic N, (c) pyrrolic N, and (d) pyridinic N-oxide. The gray, blue, red and
white balls stand for C, N, O and H atoms, respectively. Both parallel adsorption and vertical adsorption were calculated.
catalytic activity, and graphite N is the key determinant for
Conclusions
impelling the reaction. Moreover, the NGꢀ1 catalyst displays
remarkable activity toward reduction of halogen substituted
nitroarenes without undergoing any dehalogenation, affording
the corresponding products in high yields. We have succeeded
to detect the reaction intermediate by using the GCꢀMS
measurement, and proposed the reaction mechanism. It is worth
mentioning, DFT calculations reveal that N species decorated
In conclusion, we have synthesized four kinds of NG catalysts
by a oneꢀstep hydrothermal reaction of graphene oxide with
urea and thermal treatment. The NGꢀ1 catalyst shows high
catalytic performance during the reduction of nitroarenes in
water by a small amount of NaBH in water. Our results reveal
that different type of Nꢀdoping in NG significantly affect the
4
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Jou6Grnal Name
into graphene matrix, especially the quaternary ones exhibits 18 L. Shang, B. Zeng and F. Zhao, ACS Appl. Mater. Inter., 2015,
the lowest adsorption energy and the longest NꢀO bonds, 122ꢀ128.
indicating the highest catalytic activity. The catalyst can be 19 Z. Y. Sui, Y. N. Meng, P. W. Xiao, Z. Q. Zhao, Z. X. Wei and B. H.
simply and efficiently used for eight consecutive runs without Han, ACS Appl. Mater. Inter., 2015, , 1431ꢀ1438.
significant loss of activity. Further work is in progress to extend 20 X. K. Kong, Z. Y. Sun, M. Chen and Q. W. Chen, Energ. Environ.
7,
7
carbon catalysis for other organic molecule transformation
reactions.
Sci., 2013,
21 Y. Wang, Y. Y. Shao, D. W. Matson, Li, J. H. and Lin, Y. H. ACS
Nano, 2010, , 1790ꢀ1798.
22 X. L. Li, X. R. Wang, L. Zhang, S. W. Lee and H. J. Dai. Science
008, 319, 1229ꢀ1232.
6, 3260ꢀ3266.
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Acknowledgements
2
This work was supported by National Natural Science
Foundation of China (Nos. 21202203, 21576289 and
2
2
2
2
2
2
2
3
3 F. CervantesꢀSodi, G. Csanyi, S. Piscanec and A. C. Ferrari, Phys.
Rev. B: Condens. Matter Mater. Phys., 2008, 77, 165427.
4 Y. Y. Shao, J. H. Sui, G. P. Yin and Gao, Y. Z. Appl. Catal. B-
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1322609), Science Foundation Research Funds Provided to
New Recruitments of China University of Petroleum, Beijing
No. YJRCꢀ2013ꢀ31), Science Foundation of China University
of Petroleum, Beijing (No. 2462015YQ0306,
462014QZDX01), Thousand Talents Program and National
Highꢀtech R&D Program of China (863 Program, No.
015AA034603). The authors thank National Natural Science
Foundation of China for financial support (Grant Nos:
1203174), Natural Science Foundation of Jilin Province
No.20130522141JH, 20150101012JC). We are grateful to
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Computing Center of Jilin Province and Computing Center of
Jilin University for essential support.
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A graphic abstract
Four kinds of nitrogen-doped graphene (NG) as metal-free catalysts are synthesized
by a one-step hydrothermal reaction and thermal treatment using graphene oxide and
urea as precursors. It is found that the reduction of nitroarenes can be catalyzed by
4
using low NG loading and small amount of NaBH in water with high yield. The type
of nitrogen species in NG shows an important effect on the reduction reaction. The
NG catalyst containing the most graphite N shows the highest catalytic activity during
reduction of nitroarenes, which demonstrates that the graphite N of NG displays a key
role in impelling this reaction. It is worth to note that no dehalogenation phenomenon
occurs during reduction process for halogen substituted nitroarenes in contrast to
conventional metal catalysts. In addition, the NG catalyst can be simply recycled and
efficiently used for eight consecutive runs with no significant decrease in activity.